GPS involves a constellation of 24 satellites placed in orbit about the earth by the United States Department of Defense. Each satellite continuously broadcasts a GPS signal. This GPS signal contains an L-band carrier component (L1) transmitted at a frequency of 1.575 GHz. The L1 carrier component is modulated by a coarse acquisition (C/A) pseudo random (PRN) code component and a data component.
The PRN code provides timing information for determining when the GPS signal was broadcast. The data component provides information such as the satellite's orbital position. The carrier component allows a receiver to easily acquire the GPS signal.
Position determination using Conventional GPS is well known in the art. In Conventional GPS, a receiver makes ranging measurements between an antenna coupled to the receiver and each of at least four GPS satellites in view. The receiver makes these measurements from the timing information and the satellite orbital position information obtained from the PRN code and data components of each GPS signal received. By receiving four different GPS signals, the receiver can make fairly accurate position determinations.
However, Conventional GPS only allows a user to determine his actual location to within tens of meters. However, this is not suitable for applications such as attitude determination for moving vehicles which requires extreme precision.
A more accurate version of GPS is Ordinary Differential GPS. Position determination using Ordinary Differential GPS is also well known in the art. It involves the same kind of ranging measurements as are made with Conventional GPS, except that a ground reference receiver at a precisely known location is utilized. Ideally, satellite ranging errors will affect the position determinations made by the user's receiver in the same way as they will the position determinations made by the nearby ground receiver. Since the location of the ground receiver is already known, the ground receiver can compare the position determination it has calculated with the actual known position. As a result, the ground receiver can accurately detect ranging errors.
From these errors, the ground receiver can compute suitable corrections which are transmitted by data link to the user's receiver. The user's receiver can then apply the corrections to its own ranging measurements so as to provide accurate real time position determinations.
However, even with Ordinary Differential GPS, the position determinations are only accurate to within several meters. Since, as indicated earlier, attitude determination must be extremely accurate, extending Ordinary Differential GPS to attitude determination is not feasible.
An extremely accurate form of GPS is Carrier Phase Differential GPS. This form of GPS utilizes the 1.575 GHz carrier component of the GPS signal on which the PRN code and the data component are superimposed.
Carrier Phase Differential GPS involves generating position determinations based on the measured phase differences at two different antennas for the carrier component of a GPS signal. However, this technique initially requires determining how many integer wavelengths of the carrier component exist between the two antennas at a particular point in time. This is called integer ambiguity resolution.
A number of approaches currently exist for integer ambiguity resolution. However, all of them suffer from problems which render them unfit for applications requiring extremely precise attitude determinations.
One approach is Integer Searching using redundant measurements. This involves receiving more than the standard four GPS signals in order to sort out the correct combination of integer ambiguities. The different combinations of integer candidates are systematically checked against a cost function until an estimated correct set is found. However, for antenna separations of just a few meters, the checked combinations can number in the hundreds of millions. As a result, this approach has a propensity to arrive at wrong solutions. Furthermore, the configuration of the constellation of GPS satellites can only guarantee that four satellites will be in view at any given time. Therefore, any application requiring attitude determinations at any given time must not rely on redundant satellites for reliable resolution of the integer ambiguities.
Another approach is Narrow Correlator Spacing. This technique involves using the PRN code of the GPS signal to bound the integer ambiguities. However, a significant amount of the time it can yield position determination errors of as much as several meters. This does not provide the kind of consistency which is required in attitude determinations for moving vehicles.
Still another approach is Dual Frequency Wide-Laning. This approach also utilizes a second GPS signal broadcast by each satellite. This second GPS signal has an L-band carrier component (L2) transmitted at a frequency of 1.227 GHz. The L2 carrier component and the L1 carrier component are differenced so as to form a signal having an effective wavelength that is much longer than either of the two carrier components. From this signal, it is relatively easy to resolve the integer ambiguities. However, the L2 component is not available for civilian use. Although the denial of the second carrier component can be countermeasured with cross correlation technology, the performance of this type of technology is unproven and very expensive to implement.
One successful approach to integer ambiguity resolution is motion-based and has been utilized in static surveying applications. This approach involves taking a number of phase measurements while the user's antenna and the reference antenna are stationary. These phase measurements are made over a period of about 15 minutes. The phase measurements made during the slowly changing geometry of the GPS satellites will reveal the integer ambiguities. But, in many applications requiring attitude determination, it would be impractical to require the user's antennas to remain stationary for 15 minutes while the integer ambiguities are resolved.
Several motion-based approaches for integer ambiguity resolution have specifically been proposed for attitude determination. They involve placing antennas at various points around a spacecraft or airplane. The integer ambiguities can be resolved with rotation of the space craft or aircraft and taking several phase measurements.
However, one of these methods is restricted to rotation about multiple coordinate axes. This approach is not practical since, for example, simple rotation of an aircraft on the runway prior to takeoff is not sufficient to resolve the integer ambiguities in this method.
Another one of these methods is limited to small angle rotation of the moving vehicle with respect to the GPS satellite geometry. However, large angle rotation provides significantly more information for resolving the integer ambiguities. Since the small angle rotation method is not able to take advantage of such information, it does not provide as accurate a resolution of the integer ambiguities as would an approach utilizing large angle rotation.
In still another of these methods, the integer ambiguities can only be resolved if the airplane or spacecraft has a constant rate of rotation. This method is therefore unnecessarily limiting.